Skip to main content

Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

See other formats

VALDEZ AND SHOUP GLACIERS                              253
Former Expansion. There is proof of former expansion of Shoup Glacier from striae, roches moutonnees, etc.; but so far as we have evidence, the recent history has been one of retreat, with the exception of the slight advance mentioned above. Without knowledge of this recent forward swing, the fact of the advance was established in 1909 by the discovery of a recent moraine along the southwestern margin of the glacier. For the first half mile back from the shore, there was a minute morainic ridge winding up and down the hillside. It was from five to twenty feet from the glacier margin and was made up of angular fragments left by the receding glacier. There was wood in the debris and the stones at the edge lay upon dead shrubs whose neighbors were still growing. In some places 'there was a depression between the moraine and the glacier. Evidently a slight advance had interrupted the recession and the width of the barren zone indicated the amount of retreat since this recent advance.
During the more expanded ancient condition of the glacier, a group of barren rock Trills farther north had been scraped clean of all soil and eroded into a series of roches moutonnees forms and rock basins, several of which contained lakes. The ice had retreated from these barren hills not long before the slight advance alluded to, but vegeta-lion was finding a foothold slowly. Across this group of barren hills the small recent moraine extended in a sinuous course, and between it and the ice there was as yet almost no vegetation. Beyond these rock hills the area of recent overriding could be traced •for several miles by the narrow barren zone between the little moraine and the glacier.
Evidence of the amount of tune that has elapsed since the earlier, much more expanded position of Shoup Glacier is found in the distribution of forest on the neighboring hills. Willow, alder and cottonwood, mature, but perhaps not more than twenty or thirty years old, grow almost up to the very edge of the glacier. Scattered spruces are found ^dthin a mile or so of the glacier, but there is no mature coniferous forest at the ice front, ,as in the case of Columbia Glacier. Outside Shoup Bay, near sea level, scattered spruce •extends up Valdez Fiord as far as Valdez, but higher on the mountain slopes none is iound beyond the entrance of Shoup Bay. There is more spruce on the south than on north side of Port Valdez, and on the west than on the east side of Shoup Bay.
During the maximum of glaciation, when the extended Shoup Glacier received a tributary from Canyon Creek, the main Shoup ice stream over-deepened its valley so much that it left Canyon Creek hanging 1000 feet, though its erosion did not extend so far •as to remove the ledges beneath the ice fall where the glacier front now rests; nor did it succeed in completely erasing the three, alternating, truncated spurs that project into the bay. When Shoup Glacier was tributary to the main Port Valdez Glacier, however, that larger ice tongue deeply eroded the main fiord, leaving the Shoup valley itself hanging 480 feet. This discordance, masked by the waters of the bay, is clearly shown in Fig. 26 where the greatest depth inside Shoup Bay is seen to be 45 fathoms, while the depth outside, in Port Valdez, 115 to 142 fathoms. There is a striking contrast between this submerged, hanging Shoup Glacier valley, the largest tributary of the great glacier that formerly extended down Port Valdez from the Valdez Glacier and Lowe River valleys, and the visibly hanging valleys of the smaller glacier tributaries, whose positions are indicated in Figure 26 by the letter h. In the mouth of each of these hanging valleys a steep-sided, post-glacial gorge has been cut, or a foaming waterfall descends, as from Canyon Creek. The different levels of the hanging valleys, and the highly perched Canyon Creek valley, hanging with reference to Shoup Bay, as that hangs with refer-